U.S. patent number 6,090,343 [Application Number 09/047,635] was granted by the patent office on 2000-07-18 for triphasic composite and method for making same.
This patent grant is currently assigned to Rutgers University. Invention is credited to Bernard H. Kear, Larry E. McCandlish, Rajendra K. Sadangi, Oleg Voronov.
United States Patent |
6,090,343 |
Kear , et al. |
July 18, 2000 |
**Please see images for:
( Certificate of Correction ) ** |
Triphasic composite and method for making same
Abstract
A method for fabricating a triphasic composite such as a
WC/Co/diamond composite with a high volume fraction of diamond in a
WC/Co matrix. The method involves sintering of a WC/Co powder
compact to develop a porous preform, which displays some rigidity
and strength, infiltrating the porous preform with a controlled
distribution of carbon, and high pressure/high temperature
treatment of the carbon-containing WC/Co preform to transform the
carbon to diamond. The distribution of diamond in the composite can
be functionally graded to provide a WC/Co core and a
diamond-enriched surface, wherein all three phases form an
interconnected structure in three dimensions. Such a tricontinuous
structure combines high strength and toughness with superior wear
resistance, making it attractive for applications in machine tools
and drill bits.
Inventors: |
Kear; Bernard H. (Whitehouse
Station, NJ), Sadangi; Rajendra K. (Highland Park, NJ),
McCandlish; Larry E. (Highland Park, NJ), Voronov; Oleg
(Highland Park, NJ) |
Assignee: |
Rutgers University (Piscataway,
NJ)
|
Family
ID: |
21917846 |
Appl.
No.: |
09/047,635 |
Filed: |
March 25, 1998 |
Current U.S.
Class: |
419/45; 419/11;
419/14; 419/17; 419/18; 419/27; 419/48; 419/54; 419/55 |
Current CPC
Class: |
B22F
3/26 (20130101); C22C 1/051 (20130101); C22C
1/056 (20130101); C22C 26/00 (20130101); B22F
9/026 (20130101); B22F 1/0044 (20130101); C22C
1/055 (20130101); B22F 9/026 (20130101); C22C
1/055 (20130101); B22F 1/0044 (20130101); C22C
1/051 (20130101); B22F 3/26 (20130101); B22F
2005/001 (20130101); B22F 2998/00 (20130101); B22F
2999/00 (20130101); C22C 2026/001 (20130101); B22F
2998/00 (20130101); B22F 2998/00 (20130101); B22F
2999/00 (20130101); B22F 2999/00 (20130101); B22F
2999/00 (20130101) |
Current International
Class: |
B22F
3/26 (20060101); C22C 1/05 (20060101); B22F
003/12 (); B22F 003/26 () |
Field of
Search: |
;419/11,14,17,18,27,45,48,54,55 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Other References
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Jnl of Mats. Science vol. 24 (1989) pp. 942-950. .
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Designs: Part 1--Development of a PCV Cutting Force Model, JPT
(Aug. 1989) pp. 797-849. .
Sani, "Further Developments in PCD Tipped Drills" IDR 194, pp. 6-7.
.
Clark et al. "The Use of PCD for Petroleum and Mining Drilling",
Ultrahard Materials Technical Conference, (1998) pp. 349-362. .
Spur et al., Therm-mechanical Stress Cycle Tests on PCD Cutting Too
Materials. Industrial Diamond Review (Jan. 1997) pp. 27-34. .
Kear et al., Chemical Processing and Properties of Nanostructured
WC-CO Materials, Nanostructured Materials vol. 3 (1993) pp. 19-30.
.
Voronov et al. "Diamond Compacts", Diamond Material Proceedings
III, vol. 93-17, pp. 1018-1024. .
Yakolev et al., "The Hydrogen Concentration for the Graphite and
Diamond Growth CH.sub.4 as Species, P=1*10.sup.-3 -5*10.sup.3 Mpa,
T=500-1000 K." Diamond Material Proceedings III, vol. 93-17 of the
electrochemical Society, (1993) pp. 9-16. .
Yakolev et al., The Equilibrium constants of the Reactions for
Obtaining Graphite and Diamond Using the Napthalene Resolution at
High Pressure (P<5.0 Gpa, 300<T<1000 K), Diamond Materials
IV, Proc. vol. 95-4 of the Electrochemical Society (1995) pp.
100-105. .
Voronov et al., "High Purity Diamond Layers Growing on the Diamond
Substratum by Using the hyrocarbon Decomposition Reactions under
High Pressure" Diamond Materials, V, Proc. vol. 97-32 of the
Electrochemical Society (1997) pp. 20-24. .
Voronov et al., "Nucleation of Diamond Crystals from the SP.sup.2
and SP .sup.3 Hydrocarbons Decomposed Under high Pressure" Diamond
Materials V, Proc. vol. 97-32 of the Electrochemical Society (1997)
pp. 197-200c. .
Vereshchagin et al., Synthesis of Balls-type Diamond DAN SSSR,
(1967) 172 p. 76 in L. Vereshchagin "Synthetic Diamonds and
Hydrostatic Extrusion Selected Papers" General Editorial Board for
Foreign Language Publications, Moscow. USSR, 1987) pp. 73-74. .
Vereschagin et al., "Synthesis Carbonado-Type Diamonds" DAN SSSR,
(1969), 185, p. 555 L. Vereshchagin"Synthetic Diamonds and
Hydrostatic Extrusion Selected Papers" General Editorial Board for
Foreign Language Publications, Moscow. USSR, 1987) pp. 75-77. .
Vereschagin et al., "Microcrystalline Diamond Material and Methods
for Growing It into bodies with Predetermined Shape" L.
Vereshchagin"Synthetic Diamonds and Hydrostatic Extrusion Selected
Papers" General Editorial Board for Foreign Language Publications,
Moscow. USSR, 1987) pp. 83-86. .
Vereshchagin et al., "thermal Stability of Polycrystalline Diamond
and Borazon Sintered to Metal-Ceramics Binders at Pressures up to
50kbar", L. Vereshchagin"Synthetic Diamonds and Hydrostatic
Extrusion Selected Papers" General Editorial Board for Foreign
Language Publications, Moscow. USSR, 1987) pp. 118-121. .
Vereschchagin et al. Development and Performance Tests of
ASPK-Tipped Drill Bits, L. Vereshchagin"Synthetic Diamonds and
Hydrostatic Extrusion Selected Papers" General Editorial Board for
Foreign Language Publications, Moscow. USSR, 1987) pp. 171-173.
.
Vereshchagin et al. "Application of ASPK-Diamond Drill Bits for
Borehole Surveyings" L. Vereshchagin"Synthetic Diamonds and
Hydrostatic Extrusion Selected Papers" General Editorial Board for
Foreign Language Publications, Moscow. USSR, 1987) pp. 177-180.
.
Vereshchagin et al.,Sinking Prospecting Holes in Hard rocks with
ASPK-Tipped Drill Bits, L. Vereshchagin"Synthetic Diamonds and
Hydrostatic .
Extrusion Selected Papers" General Editorial Board for Foreign
Language Publications, Moscow. USSR, 1987) pp. 180-182. .
Vereshchagin et al.,Comparative Drilling Tests of ASPK-Tipped and
hard-Faced Drill Bits, L. Vereshchagin"Synthetic Diamonds and
Hydrostatic Extrusion Selected Papers" General Editorial Board for
Foreign Language Publications, Moscow. USSR, 1987) pp.
182-186..
|
Primary Examiner: Jenkins; Daniel J.
Attorney, Agent or Firm: Schwarz; Paul A. Buchanan
Ingersoll
Parent Case Text
This application claims benefit of Provisional Appln. 60/041,694
filed Mar. 25, 1997.
Claims
What is claimed is:
1. A method for fabricating a tricontinuous composite having three
material phases which are three-dimensionally interconnected, the
interconnected material phases including a superhard phase
material, a hard phase material, and a binder phase material, the
superhard phase material forming approximately 10-80 volume percent
of an exterior surface of the composite, the method comprising the
steps of:
providing a hard phase material and a binder phase material as a
porous preform of an article;
infiltrating the porous preform with a predetermined quantity of at
least one precursor of a superhard phase material;
transforming the precursor to the superhard phase material.
2. The method according to claim 1, wherein the step of providing
the hard and binder phase materials as a porous preform includes
the steps of:
providing a powder compact comprised of the hard and binder phase
materials; and
partially sintering the powder compact to produce the porous
preform.
3. The method according to claim 2, wherein the partial sintering
step is performed at a temperature approximately ranging between
800.degree. C. and 1450.degree. C.
4. The method according to claim 1, wherein the porous preform has
a uniform, interconnected pore structure.
5. The method according to claim 1, wherein the predetermined
quantity of the at least one precursor is selected to cause the
precursor to be uniformly distributed throughout the porous
preform.
6. The method according to claim 1, wherein the predetermined
quantity of the at least one precursor is selected to cause the
precursor to be gradiently distributed throughout the porous
preform.
7. The method according to claim 1, wherein the hard phase material
is selected from the group consisting of WC, SiC, B.sub.4 C,
Cr.sub.3 C.sub.2, VC, TaC, NbC, HfC, and mixtures thereof, the
binder phase material is selected from the group consisting of Co,
Ni, Cr, Fe, Mn, and mixtures thereof, and the precursor is a
material selected from the group consisting of amorphous carbon,
graphitic carbon, boron nitride, and mixtures thereof.
8. The method according to claim 7, wherein the superhard phase
material comprises one of diamond, cubic BN, boron carbonitride,
mixtures of diamond and cubic BN, or mixtures of diamond and boron
carbonitride.
9. The method according to claim 1, wherein the hard phase material
comprises WC, the binder phase material comprises Co, and the
precursor is a material selected from the group consisting of
amorphous carbon and graphitic carbon.
10. The method according to claim 9, wherein the superhard phase
material comprises diamond, the diamond and WC phases forming
approximately 50-97 volume percent of the composite and the Co
phase forming a balance of the composite.
11. The method according to claim 10, wherein each of the material
phases has a grain size which approximately ranges between 0.005
microns and 100 microns.
12. The method according to claim 10, wherein each of the material
phases has a grain size which is less than about 0.1 microns.
13. The method according to claim 1, wherein each of the material
phases has a grain size which approximately ranges between 0.005
microns and 100 microns.
14. The method according to claim 1, wherein each of the material
phases has a grain size which is less than about 0.1 microns.
15. The method according to claim 1, wherein the precursor material
of the superhard phase material is in a gaseous form and the
infiltrating step includes infiltrating the porous preform with a
predetermined amount of the gaseous precursor of the superhard
phase material.
16. The method according to claim 1, wherein the precursor material
of the superhard phase material is in a liquid form and the
infiltrating step includes infiltrating the porous preform with a
predetermined amount of the liquid precursor of the superhard phase
material.
17. The method according to claim 1, wherein the precursor material
of the superhard phase material is in a solid form and the
infiltrating step includes forcing the predetermined amount of the
solid precursor element of the superhard phase material into the
porous preform using pressure.
18. The method according to claim 1, wherein the transforming step
includes the step of hot-pressing the porous preform.
19. The method according to claim 18, wherein the hot-pressing step
is performed at a temperature approximately ranging between
1000.degree. C. and 2000.degree. C. and at a pressure approximately
ranging between 8-15 GPa.
20. The method according to claim 1, wherein the hard phase and
binder phase materials are provided as the porous preform and the
composite defines one of a machine tool, a drill bit, a wear
part.
21. The method according to claim 20, further comprising the step
of machining the porous preform to shape and size the preform into
the one of the machine tool, the drill bit, and the wear part
before the transforming step.
Description
FIELD OF THE INVENTION
This invention relates to triphasic composites useful in abrasive
wear and impact resistant applications and methods for making same.
More particularly, this invention relates to a tungsten
carbide/cobalt/diamond composite fabricated by infiltrating a
porous tungsten carbide/cobalt preform with a controlled quantity
of carbon and converting the carbon disposed within the preform to
diamond using hot-pressing.
BACKGROUND OF THE INVENTION
Polycrystalline diamond has greater impact resistance than single
crystal diamond. This is because polycrystalline diamond is made up
of randomly oriented grains which do not provide paths for cleavage
crack propagation. In contrast, a single cleavage crack can rapidly
propagate across a single crystal diamond. For these reasons,
polycrystalline diamond is favored over single crystal diamond in
many commercial applications.
Unfortunately, the impact resistance of polycrystalline diamond is
still relatively low. This is due to diamond's high elastic
modulus. This is a problem in some applications because
polycrystalline diamond wears by micro-fracture and spalling, and
not by atomic shearing.
The relative brittleness of polycrystalline diamond has been
addressed in the prior art. The first commercially available
polycrystalline diamond products were composite compacts comprised
of a metallic backing layer bonded directly to a diamond layer, as
shown in U.S. Pat. No. 3,745,623. The most common form of this
composite compact comprised a planar disc of polycrystalline
diamond grown directly onto a pre-cemented disc of tungsten
carbide/cobalt (WC/Co) during hot pressing.
Substrate-supported polycrystalline diamond composites possess a
number of limitations. First, polycrystalline diamond tool designs
are limited by substrate-supported polycrystalline diamond
configurations. There are many conceivable uses for polycrystalline
diamond tools that are difficult or impossible to implement with a
substrate-supported polycrystalline diamond composite. These uses
include rotary tools like miniature grinding wheels and drills
which are constructed symmetrically about a line and have working
faces that are subjected to tangential forces. Although some work
has been done to adapt substrate-supported polycrystalline diamond
composites to such uses (see for example U.S. Pat. No. 4,218,999
which describes a rotary tool comprised of a cylinder of
polycrystalline diamond grown around a core of pre-cemented
carbide), rotary tools are generally not commercially possible to
implement with substrate-supported polycrystalline diamond.
Second, the pre-cemented carbide substrate of a substrate-supported
polycrystalline diamond composite has a higher coefficient of
thermal expansion than the polycrystalline diamond of the
composite. Because the bond between the diamond layer and the
carbide substrate is formed when both materials are at a
temperature ranging between 1500.degree. C. and 2000.degree. C.,
high stresses are created when the composite compact cools to
ambient temperature.
Third, the diamond layer thickness in a substrate-supported
polycrystalline diamond composite is limited by "bridging" of the
fine diamond powder used in making the polycrystalline diamond.
Bridging is a phenomenon which occurs when fine powders are pressed
from multiple directions. During pressing the individual particles
in the pressed fine powder tend to stack up and form arches or
bridges which prevent the full pressing pressure from reaching the
center of the powder compact. When 1 micron diamond powder is used
to make a polycrystalline diamond body having a thickness greater
than about 0.06 inches, the diamond towards the center of the piece
is typically not compacted as densely as the exterior portions of
the piece. This pressing density gradient can result in cracking
and chipping of the polycrystalline diamond layer.
Other polycrystalline diamond composites have been described for
use as wear resistant cutting elements. U.S. Pat. No. 3,850,053
discloses a method for making a cutting tool blank by placing a
graphite disc in contact with cemented WC/Co and simultaneously
exposing them both to diamond forming temperatures and pressures.
U.S. Pat. No. 4,525,178 discloses a composite material that
includes a mixture of individual diamond crystals and pieces of
pre-sintered cemented carbide. The mixture is heated and
pressurized to create intercrystalline bonds between diamond
crystals, and chemical bonds between diamond crystals and
pre-sintered cemented carbide pieces. U.S. Pat. No. 5,128,080
describes a method for making a diamond-impregnated carbide. The
method comprises liquid phase sintering a green body fabricated
from a WC/Co/graphite powder blend and transforming the graphite in
the sintered mass to diamond using hot-pressing (high pressure/high
temperature). The particle size of each of the phases in the
consolidated product was in the range 0.3-100 microns. It was not
possible, however, to make a composite having phases which each
have a grain size less than 0.2 microns. The relatively large size
of the resulting diamond particles can result in an easy crack
propagation path in the composite. Moreover, volume fractions of
diamond greater than 25 volume percent could not be incorporated
into the blend because the carbon segregated from the WC and Co due
to large differences in their densities.
Thermal expansion mismatch stresses exist between the diamond
facing and the supporting WC/Co substrate in prior art composites.
Such stresses can adversely affect the bonding of the diamond to
the substrate, leading to spalling under typical service
conditions.
Recent developments in the synthesis and consolidation of
submicron-grained WC/Co powder has resulted in higher hardness and
compressive strength in the fully sintered material. Utilizing such
a material to produce a WC/Co/diamond composite opens new
opportunities to design and manufacture a new generation of
superhard tool materials.
Accordingly, there is a need for an improved triphasic composite
and method for making same that substantially overcomes the
problems and disadvantages of the prior art.
SUMMARY OF THE INVENTION
A method for fabricating a tricontinuous composite comprising three
material phases which are three-dimensionally interconnected and
include a superhard phase material forming approximately 10-100
volume percent of an exterior surface of the composite, a hard
phase material, and a binder phase material. The method comprises
providing a hard phase material and a binder phase material as a
porous preform of an article. Next, the porous preform is
infiltrated with a predetermined quantity of at least one precursor
of a superhard phase material. The precursor is then transformed to
the superhard phase material.
BRIEF DESCRIPTION OF THE DRAWINGS
The advantages, nature and various additional features of the
invention will appear more fully upon consideration of the
illustrative embodiments described in detail below, considered
together with the accompanying drawings. In the drawings:
FIG. 1 is a block diagram depicting the steps of the method of the
present invention;
FIGS. 2A-2B are schematic representations of conventional stud
inserts for roller cone drill bits;
FIG. 2C is a schematic representation of a stud insert for a roller
cone drill bit made according to the present invention;
FIG. 3A is a schematic representation of a conventional insert for
a drag drill bit;
FIG. 3B is a schematic representation of an insert for a drag drill
bit made according to the present invention;
FIG. 4 shows a porous WC/Co preform produced from as-synthesized
nanophase WC/Co powder;
FIG. 5 shows as-synthesized nanophase WC/Co powder after about 1/2
hour of milling;
FIG. 6 shows re-agglomerated, mechanically milled as-synthesized
nanophase WC/Co powder;
FIG. 7 shows a TGA trace of an infiltrated WC/15 wt. % Co
preform;
FIGS. 8A and 8B are scanning electron micrographs of a triphasic
composite made according to the method of the present
invention;
FIGS. 9A and 9B are Raman spectra of a triphasic composite made
according to the method of the present invention;
FIG. 10 shows a TGA trace of infiltrated WC/15 wt. % Co powder;
FIGS. 11A and 11B are Raman spectra of a triphasic composite powder
made according to the method of the present invention; and
FIGS. 12A and 12B are scanning electron micrographs of a triphasic
composite powder made according to the method of the present
invention.
It should be understood that the drawings are for purposes of
illustrating the concepts of the invention and are not to
scale.
DETAILED DESCRIPTION OF THE INVENTION
A method for making a triphasic composite is described herein. The
triphasic composite comprises three polycrystalline material phases
which are interconnected in three dimensions, thus, forming a
"tricontinuous" structure. The three polycrystalline material
phases include a superhard phase material, a hard phase material,
and a binder phase material. The superhard phase material may
include diamond, cubic boron nitride (BN), boron carbonitride,
mixtures of diamond and cubic BN, mixtures of diamond and boron
carbonitride, or other superhard phase materials. The hard phase
material may include tungsten carbide (WC), silicon carbide (SiC),
boron carbide (B.sub.4 C), chromium carbide (Cr.sub.3 C.sub.2),
vanadium carbide (VC), tantulum carbide (TaC), niobium carbide
(NbC), hafnium carbide (HfC), mixtures thereof, or other hard phase
materials. The binder phase material may include cobalt (Co),
nickel (Ni), chromium (Cr), iron (Fe), manganese (Mn), or mixtures
thereof. The superhard phase material may form approximately 10-100
volume percent of the exterior surface of the composite.
Referring to FIG. 1, block A of the method consists of providing a
porous preform. The preform may have a shape of a desired article.
The porous preform is composed of at least one of the earlier
described hard phase materials and at least one of the earlier
described binder phase materials. The porous preform is produced by
partially sintering a powder compact (produced by conventional
pressing or extrusion methods) composed of the selected hard and
binder phase materials. When partially sintered, the hard phase and
binder phase particles form a bicontinuous structure that displays
some rigidity and strength. At this stage of the method, additional
precision shaping of the preform may be carried out because the
porous material is easily machined.
In block B of the method, the porous preform is infiltrated with a
predetermined quantity of one or more precursor materials of the
earlier described superhard phase materials. Examples of these
precursor materials include carbon (precursor material for diamond)
and boron nitride (precursor material for cubic BN). Infiltration
may be accomplished using impregnation of solid precursors,
infiltration of liquid precursors or infiltration using chemical
vapor precursors. Infiltration enables different types of low
density materials or reactive compounds to be distributed within
the porous preform. The size, shape and distribution of the
infiltrated material can be controlled by increasing or decreasing
the quantity of the precursor material, or by increasing or
decreasing the length of time of infiltration. Accordingly, a
gradient distribution of the precursor material can be provided
through the porous preform such that the amount of the precursor
material increases gradually from the interior or core of the
preform to the exterior of the preform. The precursor material can
also be provided in a uniform distribution throughout the
preform.
In block C of the method, the precursor material is disposed within
the porous preform is transformed to the superhard phase material.
Accordingly, a tricontinuous fully sintered composite having three,
three-dimensionally interconnected polycrystalline material phases
is realized. The polycrystalline material phases of the composite
will include a superhard phase material, a hard phase material, and
a binder phase material. The distribution of the superhard phase
material in the fully sintered composite will increase gradually
from the interior or core of the composite to the exterior of the
composite if the precursor material was gradiently distributed
through the porous preform. The superhard phase material introduced
into the surface of the composite can range from about 10 to 75
volume percent. The distribution of the superhard phase material in
the filly sintered composite will be uniform from the interior to
the exterior of the composite if the precursor material was
uniformly distributed through the porous preform.
Transformation of the precursor material to the superhard phase
material may be accomplished using a high pressure/high temperature
(HPHT) process. In the HPHT process, the precursor infiltrated
porous preform is introduced into a resistively-heated high
pressure cell and subjected to a pressure of about 5-15 GPa at a
temperature of about 1000-2000.degree. C.
The method of the invention is especially useful for producing a
functionally graded, tricontinuous nanophasic WC/Co/diamond
composite. The WC/Co/diamond composite comprises a WC/Co core and a
diamond-enriched surface. All three phases of the composite each
have a grain size less than 0.1 micron (less than 100 nanometers)
and form an interconnected structure in three dimensions. The
WC/Co/diamond composite combines high strength and toughness with
superior wear resistance, making it attractive for applications in
machine tools and drill bits.
The WC/Co/diamond composite is made in the above described method
by partially sintering a WC/Co powder compact preferably pressed
from a submicron or "nanophase" WC/Co starting powder. Partial
sintering may be performed at a temperature approximately ranging
between 800.degree. C. and 1450.degree. C., and preferably between
800.degree. C. and 1150.degree. C. The nanophase WC/Co powder can
be obtained from Nanodyne Corporation.
The nanophase WC/Co powder is produced in a well known spray
conversion process (SCP). The SCP process involves preparing an
aqueous solution of mixed tungsten and cobalt salts which provides
a starting solution of a fixed composition. The solution is then
spray dried to form an amorphous precursor powder consisting of a
uniform mixture of salts. The precursor powder is converted into
the nanophase WC/Co product powder using a fluid-bed thermochemical
conversion process which involves pyrolysis, reduction and
carburization of W to WC. During the carburization of W to
WC, a controlled carbon activity gas stream of CO/CO.sub.2 or
CO/H.sub.2 is used to fully carburize W without producing excess
graphitic carbon. The nanophase WC/Co powder has a spherical-shell
morphology similar to that of a spray-dried powder (typically 5-50
microns in diameter and 2-10 micron wall thickness). However,
unlike a typical spray-dried powder, the nanophase WC/Co powder
product has a high degree of interconnected porosity. This provides
the WC/Co powder particles with a sponge-like structure, with the
walls of the sponge composed of equiaxed grains of well-bonded WC
and Co nanoparticles.
The partially sintered porous WC/Co preform obtained from the
nanophase WC/Co powder compact is infiltrated with a carbon
precursor material. Infiltration of the porous WC/Co preform is
preferably accomplished by chemical vapor infiltration of amorphous
or graphitic carbon supplied at low pressure using gaseous
hydrocarbons, such as methane, ethane, or ethylene. Infiltration
may also be achieved by liquid phase infiltration at high pressure
using liquid hydrocarbons, such as wax, pitch, and bitumen, or by
impregnation with carbon at high pressure using fullerenes. During
infiltration, the deposition of carbon can be controlled to give a
specific volume fraction of carbon relative to that of the original
WC and Co phases thereby providing a uniform or functional
distribution of carbon in the porous preform.
The carbon-infiltrated WC/Co preform is introduced into the
resistively-heated high pressure cell and subjected to the 5-15 GPa
pressure and 1000-2000.degree. C. temperature (the HPHP process).
Under these conditions, the cobalt promotes the transformation of
carbon to diamond inside the WC/Co. Accordingly, the highly
desirable tricontinuous nanophasic WC/Co/diamond composite is
realized. The resulting WC/Co/diamond composite comprises a diamond
polycrystal which grows through the nanostructured WC/Co
polycrystal. The diamond polycrystal rises inside the WC/Co
polycrystal and grows from the bottom to the top of the
composite.
FIGS. 2A-2C are schematic representations of "stud inserts" for
roller cone drill bits. FIG. 2A shows a conventional WC/Co insert
20 and FIG. 2B shows a conventional WC/Co insert 22 with a
polycrystalline diamond layer 24. FIG. 2C shows a functionally
graded WC/Co/diamond insert 26 made according to the method of the
present invention. The graded insert 26 has a core 28 which
contains less than 5 volume percent diamond phase material. The
volume percent of the diamond phase material gradually increases to
greater than 50 volume percent diamond phase material as you move
from the core 28 to the exterior 30 of the graded insert 26. The
insert 26 is also coated with an optional layer of polycrystalline
diamond 46. This provides about 100 volume percentage of diamond at
the surface of the insert 26. Because the insert 26 is functionally
graded, thermal expansion and modulus mismatching stresses are
avoided at the interface of the diamond layer 46 and the insert 26.
Consequently, as the diamond layer 46 wears away in service
(usually at an inclined angle), the presence of the graded
underlying insert surface reduces the wear rate and increases the
useful life of the insert 26.
The optional diamond layer 46 may be fabricated by applying a layer
of diamond grit to the carbon infiltrated insert preform prior to
the transformation step. The preform is then subjected to HTHP
carbon transformation process which bonds the diamond grit layer
(which is transformed to the polycrystalline diamond layer 46) to
the insert and transforms the infiltrated carbon to polycrystalline
diamond.
The diamond layer may also be fabricated by applying a layer of
catalyzed carbon to the carbon infiltrated insert preform prior to
the transformation step. The preform is then subjected to HTHP
carbon transformation process which transforms the infiltrated
carbon and the carbon layer to polycrystalline diamond.
FIGS. 3A and 3B are schematic representations of polydiamond
carbide inserts for drag drill bits. FIG. 3A shows a conventional
WC/Co insert 32 with a polycrystalline diamond layer 34. FIG. 3B
shows a functionally graded WC/Co/diamond insert 36 made according
to the method of the present invention. The graded insert 36 has a
core 38 which contains less than 5 volume percent diamond phase
material. The volume percent of the diamond phase material
gradually increases to about 80 volume percent diamond phase
material as you move from the core 38 to the exterior 40 of the
graded insert 36.
The following discussion sets forth the applicants'experimental
work. Porous preforms were fabricated from three different types of
starting nanophase WC/Co powders. These powders consisted of
as-synthesized powder, mechanically milled as-synthesized powder,
and solid agglomerated, mechanically milled as-synthesized
powder.
As-synthesized nanophase WC/Co powders, produced by Nanodyne Inc.,
are typically about 5-50 microns in diameter and have a
characteristic spherical-shell morphology. The thick walls of these
hollow particles are highly porous in nature and are composed of
equiaxed nanograins of WC and Co phases.
Porous WC/Co preforms were produced from as-synthesized WC/Co
powder by first separating the shell-like particles using sieving
and other known methods. Particles of the same diameter were then
poured into a mold, lightly compressed, and sintered at
850-1150.degree. C. in a reducing environment. In this temperature
range, surface diffusion dominates so that sintered junctions are
formed between the particles without a significant reduction in the
size of the compact. Consequently, the powder compact retains its
initial porosity, albeit in a somewhat coarsened form.
FIG. 4A schematically shows a single spherical shell particle 48 of
as-synthesized nanophase WC/Co powder. The particle 48 typically
measures about 10-15 microns in diameter. The wall 50 or shell of
the particle 48 is connected together by smaller pores 52.
FIG. 4B schematically shows a section of one of the porous WC/Co
preforms produced from as-synthesized nanophase WC/Co powder. The
preform was highly porous and consisted of large pores 54 define by
the shell-like particles 48, connected together by much smaller
pores 52 (within the walls 50 of the shell-like particles 48). A
similar but more irregular sintered structure was developed when
the powder compact was heavily compressed to break up many of the
shell-like particles in the mold, prior to partial sintering.
It should be noted that the preferred sintering temperature will
depend on whether or not the nanophase WC/Co powder contains
additives, such as VC or Cr.sub.3 C.sub.2, which are known grain
growth inhibitors. Since these additives reduce the incipient
melting point of the Co-rich matrix phase, partial sintering may be
achieved at temperatures .about.850.degree. C.
Porous WC/Co preforms were produced from mechanically milled
as-synthesized nanophase WC/Co powder. Mechanical milling easily
breaks up the as-synthesized WC/Co powder into finer size
fractions. The operation was carried out in an oxygen-free
environment to avoid powder contamination. This was achieved by
mechanically milling the WC/Co powder in a fluid medium of hexane
plus 10% paraffin binder under a blanket of argon. After about 1/2
hour of milling, the shell-like nanocomposite particles 48 were
reduced to fragments 56 that were about 0.1-0.3 microns in diameter
as shown in FIG. 5.
The powder fragments were cold pressed at 0.5-1.0 GPa, and then
partially sintered at 850-1050.degree. C. in flowing 2% H.sub.2
/argon mixture to produce a porous WC/Co preform. The resulting
oxygen-free porous preform had a uniform interconnected network of
fine submicron-scale pores. To avoid further contamination, the
powder was passivated with a hydrocarbon species, such as
hexane/10% paraffin mixture.
Porous WC/Co preforms were produced from solid agglomerated,
mechanically milled as-synthesized nanophase WC/Co powder.
Mechanically milled powder, reduced to fragments 56 about 0.1-0.3
micron size, as described above, can be re-agglomerated by spray
drying using a suitable binder phase, preferably a water-soluble
binder, such as polyvinyl alcohol. By making appropriate
adjustments in the spray drying parameters, which are well known to
those skilled in the art, re-agglomerated powder can be produced to
provide particles 58 with a size controllable over the 5-50 micron
size range as shown in FIG. 6.
Porous WC/Co preforms were formed by pouring the agglomerated
powder into a mold, lightly compressing the powder, and partially
sintering the powder at 850-1050.degree. C. in a 2% H.sub.2 /argon
atmosphere.
Examples of fully sintered composites composed from nanophase WC/8
wt. % Co and nanophase WC/15 wt. % Co are now described. Partially
sintered preforms of nanophase WC/15 wt. % Co exhibited higher
strengths than partially sintered preforms of nanophase WC/8 wt. %
Co. Although both types of material have their uses, WC/15 wt. % Co
has greater applicability because of its higher intrinsic fracture
resistance.
EXAMPLE 1
Nanophase WC/15 wt. % Co powder was uniaxially compacted at 50 MPa
into a 3 mm diameter.times.2 mm high sample. A floating die
configuration was used to minimize density gradients in the green
body. The compact was placed in a graphite crucible and inductively
heated to 800.degree. C. in flowing H.sub.2 to remove surface
oxides. Subsequently, the chamber was evacuated and the sample
heated to 900.degree. C. for 30 minutes. No significant dimensional
changes occurred during this pre-sintering treatment. The
pre-sintered compact was 36% dense and had sufficient strength for
handling purposes.
Chemical vapor infiltration (CVI) of the porous compact was carried
out in a controlled atmosphere thermal gravimetric analyzer (TGA).
Weight changes were recorded using a Cahn 1000 micro-balance. The
temperature of the furnace was ramped at 15.degree. C./min. up to
900.degree. C. in a flowing gas mixture (100 cc/min.) of H.sub.2
/10% CH.sub.4, and held at this peak temperature for 3hours. FIG. 7
shows a TGA trace indicating carbon pick up by chemical vapor
infiltration of a WC/15 wt. % Co preform. The corresponding weight
pick up of the sample was about 20 wt. %, which is equivalent to
about 45 vol % carbon deposited within the porous sample.
The carbon-infiltrated sample was then placed in the reaction cell
of a high pressure/high temperature (HPHT) press. The space between
the sample and graphite crucible was packed with hexagonal BN,
which acts as an insulator and pressure transmitting medium. The
porous sample was heated to .about.1600.degree. C. under a pressure
of 8 GPa in order to fully consolidate the material, and to
transform the graphite to diamond. The formation of a relatively
high volume fraction of diamond was established by scanning
electron microscopy (SEM) examination. FIG. 8A is a secondary
electron image, scanning electron micrograph showing bright areas
that represent mixtures of WC and Co, and much darker areas that
represent diamond. FIG. 8B is a back-scattered electron image,
scanning electron micrograph which confirms this WC/Co/diamond
phase distribution.
The formation of a relatively high volume fraction of diamond was
also confirmed by Raman examination. Raman microprobe spectra were
recorded from 1290-1390 cm.sup.-1 and from 1520-1620 cm.sup.-1 at
0.1 cm.sup.-1 intervals. FIG. 9A is the spectra collected at
1290-1390 cm.sup.-1. The spectra showed two peaks, one at 1329
cm.sup.-1 and the other at 1370 cm.sup.-1. The first peak
corresponds to diamond and the second peak corresponds to
disordered diamond. FIG. 9B is the spectra collected at 1520-1620
cm.sup.-1. The absence of a peak at 1580 cm.sup.-1 clearly shows
that there is no graphitic carbon in the sample. The Raman spectra
is similar in appearance to that found in a CVD-generated
microcrystalline diamond film.
EXAMPLE 2
Nanophase WC/15 wt. % Co powder was placed in a platinum boat, and
chemical vapor infiltration (CVI) of the loose powder mass was
carried out in a controlled atmosphere TGA unit. Weight changes
were recorded using a Cahn 1000 microbalance. The temperature of
the furnace was ramped at 15.degree. C./min. up to 900.degree. C.
in a flowing gas mixture (100 cc/min.) of H.sub.2 /10% CH.sub.4,
and held at this peak temperature for 3 hours. FIG. 10 shows a TGA
trace indicating carbon pick up by chemical vapor infiltration of
WC/15 wt. % Co powder. The weight pick up experienced by the sample
was about 30 wt. %, which is equivalent to about 55 vol. % of
carbon deposited within the porous powder mass.
The carbon-infiltrated sample was placed in the reaction cell of an
HPHT unit. The porous powder mass was heated to .about.1600.degree.
C. under a pressure of 8 GPa in order to fully consolidate the
material, and to transform the graphite to diamond. Raman and SEM
examination confirmed the presence of a high volume fraction of
diamond. FIG. 11A is a Raman spectra of the HPHT treated sample in
the 1290-1390 cm.sup.31 1 range. FIG. 11B is a Raman spectra of the
HPHT treated sample in the 1520-1620 cm.sup.-1 range. FIG. 12A is a
secondary electron image scanning electron micrograph of the HPHT
treated sample. FIG. 12B is a backscattered electron image scanning
electron micrograph of the HPHT treated sample.
EXAMPLE 3
Nanophase WC/15 wt. % Co powder was uniaxially compacted at 50 MPa
into a 3 mm diameter.times.2 mm high sample, and partially sintered
by heating in flowing H.sub.2 at 1000.degree. C. The resulting
porous preform was infiltrated with graphitic carbon, using a
flowing gas mixture of H.sub.2 /20% CH.sub.4 mixture at 950.degree.
C. for 1 hour. The procedure was similar to example 1, except for
the much faster kinetics of carbon deposition, due to the higher
concentration of CH.sub.4 in the gas stream, and the higher
reaction temperature. The effect of this treatment was to develop a
compositionally graded structure, in which the carbon concentration
gradually decreases from the surface to the interior of the sample.
After infiltration, the sample was heated to .about.1600.degree. C.
under a pressure of 8 GPa in order to fully consolidate the
material, and to transform the graphite to diamond. The resulting
WC/Co/diamond nanocomposite material, in which the diamond
concentration gradually diminishes from the surface to the
interior, is described as a functionally graded material, because
it combines a wear resistant diamond-enriched surface with a
strong, tough supporting core of WC/Co.
EXAMPLE 4
Nanophase WC/15 wt. % Co powder was mechanically milled using a
Union Process H-01 attritor mill. The mill was operated at 250 rpm
and the milling time was 3 hours. The charge was 100 gm of WC/Co
powder and 2000 gm of 0.6 cm diameter grinding balls. The milling
medium consisted of eskar wax dissolved in 150 cc of hexane. After
milling, about 80 gm of powder was recovered.
The milled powder was uniaxially compacted at 50 MPa into a 3 mm
diameter.times.2 mm high sample, dewaxed at 500.degree. C., and
pre-sintered at 900.degree. C. in vacuum. The resulting porous
preform was infiltrated with carbon, as in example 1. The rate of
carbon pickup was slow; only about 25 vol. % carbon was infiltrated
in 3 hours, using a H.sub.2 /10% CH.sub.4 mixture at 900.degree. C.
The infiltrated sample was HPHT pressed to consolidate and
transform the carbon to diamond.
EXAMPLE 5
Mechanically milled powder, as in Example 4, was dispersed in an
aqueous solution of polyvinyl alcohol (PVA), and spray dried in a
Yamato spray drier to form spherical agglomerates. The spray drying
solution contained 50 wt. % of WC/Co solid, 5 wt.% of PVA binder,
and 45 wt. % of water. The spray drying conditions were as follows:
inlet temperature 290.degree. C., outlet temperature 70-80.degree.
C., atomizing air pressure 2 kg/cm.sup.2 , nozzle diameter 0.15 cm,
and drying air flow rate 0.6 cm.sup.3 /min. The agglomerated powder
had a mean particle size of 60 micron.
The agglomerated powder was uniaxially compacted at 50 MPa into a 3
mm diameter.times.2 mm high sample, dewaxed at 250.degree. C., and
pre-sintered at 900.degree. C. in vacuum. The resulting porous
preform was infiltrated with carbon, as in example 1. The rate of
carbon pickup was slow; only about 25 vol. % carbon was infiltrated
in 3 hours using a H.sub.2 /10% CH.sub.4 mixture at 900.degree. C.
The infiltrated sample was HPHT pressed to consolidate and
transform the carbon to diamond.
EXAMPLE 6
Nanophase WC/15 wt. % Co powder (50 micron shell diameter, 5 micron
wall thickness, 0.05 micron grain size) was compacted on a
supporting fully dense WC/15 wt. % Co substrate. The compact was
placed in an inert gas
furnace and heated to 1050.degree. C. for 30 minutes. A porous
WC/Co sponge formed on the dense WC/Co substrate. The apparent
density of the sponge was .about.7.0 g/cm.sup.3, and open porosity
was 50%. The apparent density of the substrate was .about.14
g/cm.sup.3, and open porosity was 0%. The sample was cylindrically
shaped to 4.5 mm diameter and 3 mm height. Chemical vapor
infiltration of the porous part of the compact was carried out in a
controlled atmosphere thermal gravimetric analyzer, as in Example
1. The sample with the carbon deposited in its pores was placed in
a HPHT reaction cell and heated to 1500.degree. C. under a pressure
of 9 GPa for 10 sec. In the presence of Co, the graphite-like
carbon transformed into diamond polycrystal with a crystallite size
of 0.1-1.0 micron. The sponge-like diamond grew through the
sponge-like WC/Co.
EXAMPLE 7
The sample was prepared as in Example 6. Liquid phase infiltration
of the porous part of the compact was carried out in the HPHT
reaction cell at a pressure of 0.5 GPa and a temperature of
300.degree. C. At this temperature, the pitch melted and
infiltrated into the porous compact. The temperature was then
increased to 600.degree. C. at the same pressure in order to
carbonize the hydrocarbon. The sample was placed in a vacuum
furnace for heat treatment to 1400.degree. C. to graphitize the
carbon. The sample with the carbon deposited in its pores was
placed in a HPHT reaction cell and heated to 1000.degree. C. under
a pressure of 15 GPa for 10 sec. The graphitic carbon transformed
into diamond polycrystal, as in Example 6, but with a crystallite
size of 0.005-0.1 micron.
EXAMPLE 8
The sample was prepared as in Example 6. Impregnation of C.sub.60
fullerene into the porous part of the compact was carried out in
the HPHT reaction cell at a pressure of 1 GPa and a temperature of
300.degree. C. At this temperature the fullerene C.sub.60
impregnated the pores. The pressure was then increased to 8 GPa,
and the temperature was increased to 1200.degree. C. The fullerene
carbon transformed into diamond polycrystal, as in Example 6, but
with a crystallite size of 0.005-0.05 micron.
EXAMPLE 9
SiC powder was mixed with 15 wt. % Ni-Fe-Co-Cr eutectic alloy and
milled, as in Example 4. The milled powder was compacted and
sintered in an inert gas furnace at a temperature of 1300.degree.
C. and at ambient pressure. The fully dense substrate of
polycrystalline SiC/NiFeCoCr was placed in a graphite crucible. The
same powder was poured onto the substrate and sintered in an inert
gas furnace at a temperature of 900.degree. C. for 30 min.
SiC/NiFeCoCr sponge appeared on the SiC/NiFeCoCr substrate with
zero porosity. The open porosity of the sponge was 50%. Chemical
vapor infiltration of the porous part of the compact was carried
out as in Example 1.
The sample with carbon deposited in its pores was placed in the
HPHT reaction cell and heated to 1200.degree. C. under a pressure
of 7 GPa for 30 sec. In the presence of Ni-Fe-Co-Cr alloy, the
graphite-like carbon transformed into diamond polycrystal with a
crystallite size of 1-2 micron.
From the foregoing examples, it is apparent that both uniform and
functionally graded WC/Co/diamond nanocomposites can be produced
using the method of this invention. Of course, it should be
understood that a wide range of changes and modifications in the
preferred embodiment described above will be apparent to those
skilled in the art. It is, therefore, intended that the foregoing
detailed descriptions be regarded as illustrative rather than
limiting, and that it be further understood that it is in the
following claims, including all equivalents, that we intend to
define the scope of the present invention.
* * * * *